1. Field of the Invention
This invention relates to the acquisition of a radiographic image using a large area, direct conversion, solid state detector. More specifically, the present invention concerns an electronic solid-state image capture device for static and dynamic x-ray imaging applications. In particular, it relates to devices using a direct converter material, such as amorphous selenium, a flat panel TFT array, and solid-state charge amplifiers, such that the mode of operation prevents excessive voltage built-up across the TFT array that would be damaging to the TFT transistors.
2. Description of the Prior Art
A number of attempts have been made in the past to minimize high voltage effect or to achieve high voltage protection in image capture devices using a thin film transistor (TFT) array.
For example, in the paper by Zhao Wei, Law James, Waechter D., Huang, Z., and Rowlands J., entitled xe2x80x9cDigital Radiology using active matrix readout of amorphous selenium detectors with high voltage protectionxe2x80x9d, 1998, Med Phys 25 (4), pp. 539-549, a selenium-based x-ray imager with a special TFT dual-gate structure is disclosed which provides a high voltage protection. The dual gate structure provides the high voltage protection by forming a back channel in the TFT structure if the pixel voltage exceeds a certain potential, thus discharging the storage capacitor. Its main drawback comes from the fact that the saturation potential is controlled by deposition factors in the processing of the TFT array (i.e. ratio of oxide thickness of top and bottom gate insulators), and therefore cannot be externally controlled. The polarity of the bias voltage for the selenium layer in this example is positive high voltage.
In PCT International Application WO 96/22616 published Jul. 25, 1996, a TFT structure is described which minimizes the charge injection of the TFT switching for x-ray detectors. In this case, no mention is made about the high voltage protection, and the polarity used to bias the selenium layer is positive high voltage.
In U.S. Pat. No. 5,198,673 of Mar. 30, 1993 by Rougeot et al., a photosensitive selenium-based x-ray imager is described where the high voltage protection is provided by the presence of a second two-terminal protection device resident at each pixel location. This technique suffers from the fact that array construction is more complex, and that array yields may suffer as the pixel size is decreased since more and more circuitry must be placed on a smaller pixel size. Also, the saturation voltage of the pixel is determined by TFT array processing conditions, and therefore cannot be externally controlled.
In a paper by Lee D., Cheung L. K., and Jeromin L., entitled xe2x80x9cA new digital detector for projection radiographyxe2x80x9d, 1995, SPIE Vol. 2432, pp. 237-249, selenium-based TFT imaging system is disclosed where a thick dielectric layer is interposed between the high voltage bias electrode and the selenium layer. Thus, the high voltage protection mechanism comes from the fact that negative charges accumulate at the interface between the selenium layer and the insulator layer, thereby lowering the electric field and x-ray sensitivity of the selenium layer. However, this technique suffers from the fact that the negative charges must be eliminated prior to making a successive image, and hence this technique cannot be used for real-time imaging applications.
In U.S. Pat. No. 5,598,004 of Jan. 28, 1997 by Powell et al. and U.S. Pat. No. 5,396,072 of Mar. 7, 1995 by Schiebel et al. photoconductor-based imaging detectors are described, where one of the embodiments uses a selenium-based energy conversion layer. However, no mention is made of the high voltage protection of the TFT array.
In U.S. Pat. No. 5,528,043 of Jun. 18, 1996 by Spivey et al., a selenium-based system is described, where the active substrate uses a metal oxide semiconductor (MOS) circuit technology on silicon wafers, rather than TFT technology on glass substrates. In this technology, the design rules allow for a higher degree of integration, and therefore do allow to incorporate several transistors per pixel. This allows, for example, the use of a buffer to non-destructively read out a pixel, but does not mention whether high voltage protection of the circuit from the selenium bias is achieved.
In U.S. Pat. No. 5,436,101 of Jul. 25, 1995 by Fender et al., there is described a selenium structure which can be charged negatively by placing a blocking layer between the selenium and the substrate, but there is no mention of any high voltage protection of any element on the substrate.
Finally, in Canadian Patent Application No. 2,184,667 by Alain Jean and Bradley Trent Polischuk, published on Mar. 4, 1998, and corresponding European Application EP 0 826 983 also published on Mar. 4, 1998, a selenium multilayer structure is disclosed which, in addition to providing for real-time imaging capabilities, also leads to increased mechanical durability. However, no indication of how this structure could be used for high voltage protection is given.
It is an object of the present invention to overcome the various defects and limitations mentioned above by allowing a direct external control of the built-in voltage on the pixel capacitor.
A further object of the present invention is to achieve the desired high voltage protection by providing a photoconductor layer that can be biased negatively.
A still further object of the present invention is to fabricate a stack of layer forming a selenium p-i-n structure which allows negative bias at a very low dark current.
Other objects and advantages of the present invention will become apparent from the following description thereof.
In essence, the present invention comprises a direct conversion x-ray image electronic detector which has an n-channel active matrix thin film transistor (TFT) substrate, a coplanar selenium diode structure and a high voltage biasing electrode and in which high voltage protection is achieved by setting the high voltage biasing electrode to a negative potential and the TFT xe2x80x9coffxe2x80x9d gate voltage to a predetermined negative value, such that the TFT is essentially non-conductive. Such voltage is typically in the range of xe2x88x921V and xe2x88x9220V and preferably about xe2x88x9210V. In this regard, there will always be some TFT leakage, however the negative xe2x80x9coffxe2x80x9d gate voltage may be adjusted so as to minimize the same and render the TFT essentially non-conductive.
The saturation exposure of the x-ray image electronic detector can be adjusted by changing the negative value given to the xe2x80x9coffxe2x80x9d gate voltage. Also, an external gain control may be obtained by changing the value of the negative high voltage on the biasing electrode which is typically between 1 kV and 30 kV.
The detector should also be provided with suitable charge amplifiers such as to sink the current coming from the TFT arrays.
In a preferred embodiment, the novel x-ray electronic detector uses a selenium p-i-n multilayer converter layer as a signal current source to avoid time delays in emptying and replenishing material charge traps. As is known, the p-i-n structure is a diode with the i layer sandwiched between the p and n layers.